The present invention relates to the field of beam steering, and more specifically, to a method and a device that prevent gimbal-locking of gimbal mounts and related beam-steering devices.
In many fields it is necessary to mount a directional device on a platform so as to allow the directional device to be oriented independently of platform orientation. A device that has proven exceptionally useful for this task is the gimbal mount. A gimbal mount is basically a mounting frame having two orthogonal axes of rotation. In FIG. 1, a typical gimbal mount 10 is depicted, where a telescope 12 is mounted to a platform 14 (in FIG. 1, a raft). Telescope 12 is directly mounted to a moveable nod ring 16 that is mounted on a moveable roll ring 18, connected to platform 14. The orientation of telescope 12 can be changed by movement around nod axis 20 and around gimbal roll axis 22 of gimbal mount 10. As a result, telescope 12 can be oriented without being influenced by the orientation of platform 14.
One specific application where gimbal mounts are used is to mount a directional seeker (e.g. infrared, UV/vis) to the nose of a projectile (e.g. missile, smart-bomb, cannon/artillery shell and the such) or to track satellites using a radio-frequency antenna. In FIG. 2A, a gimbal mount 24 is used to allow seeker 26 of a projectile 28 with transparent nose cover 29 to be oriented in the direction of a moving target 32, while the relative position of moving target 32 and projectile 28 changes. Gimbal mount 24 has two rotatable axes, gimbal nod axis 34 and gimbal roll axis 36.
A serious shortcoming of a gimbal mount such as 24 occurs when the directional device, such as seeker 26, needs be directed at or in proximity of a direction 30 which is close to colinear to gimbal roll axis 36, FIG. 2B. In order for seeker 26 to remain directed at moving target 32 passing at or near direction 30, gimbal roll axis 36 must rotate quickly requiring an extremely high, often unattainable, rotational acceleration. This problem is called gimbal locking or as the keyhole problem.
The nature of the problem of gimbal locking has been fully described in U.S. Pat. No. 6,285,338, which is incorporated by reference for all purposes as if fully set forth herein. Specifically, FIG. 13 of U.S. Pat. No. 6,285,338 and the accompanying description discuss the angular speed required to track a target moving near or through a direction which is colinear with the gimbal roll axis.
To change the orientation of the directional device at a given speed, the closer the gimbal roll axis is to colinearity with the direction vector the faster the gimbal roll axis must move. In FIG. 13 of U.S. Pat. No. 6,285,338, to track a given satellite using a gimbal mounted radar antenna (the directional device), a 5° divergence requires an angular rotation of 1° sec−1. To track the same satellite, a 1° divergence requires an angular rotation of 4° sec−1 and a 0.1° divergence requires an angular rotation of 12° sec−1.
One method to overcome the problem of gimbal locking is to provide a massive gimbal mount equipped with powerful motors. For projectiles, where weight and size allowances are at a premium and, due to the disposable nature of projectiles, price reduction an advantage, this is at best an academic solution. Further, it is generally preferred that high accuracy gimbal mounts be lightweight to avoid problems associated with large moments of rotation.
Another method to overcome the problem of gimbal locking is taught in U.S. Pat. No. 6,285,338. A device is provided to reorient, by tilting, the directional device relative to the gimbal mount when a gimbal locking situation is approached. In a situation where a standard gimbal mount would have to direct a directional device with, for example, a 0.1° divergence of the gimbal roll axis from the direction vector, a gimbal mount according to the teachings of U.S. Pat. No. 6,285,338 tilts the antenna by, for example, 0.9° in an appropriate direction. This tilting reduces the magnitude of angular rotation necessary for tracking threefold. Although effective, a mechanism such as taught by U.S. Pat. No. 6,285,338 adds a level of mechanical complexity, weight and expense to a gimbal mount that often makes such a mechanism unsuitable for use in a platform, such as a projectile, where space, weight and cost are important factors.
There is a need for a lightweight and simple method to avoid gimbal locking, especially for mounting a directional device in a projectile.
As is clear to one skilled in the art, gimbal locking is not a problem unique to actual gimbal mounts, but also to related beam steering devices. Other beam steering devices shall be discussed in more detail hereinbelow. It is important to note, however, that the term “gimbal-locking” is hereinafter used to refer to actual gimbal locking of a gimbal mount as well as to the analogous problem of related beam steering devices. The description and discussion of the present invention herein will refer primarily to an actual gimbal mount rather then the more general beam-steering device. This is done exclusively for purposes of clarity and is non-limiting to the scope of the description and of the claims herein. Perusal of the description of the present invention as herein set forth allows application of the present invention to beam-steering devices other than gimbal-mounts to one of average skill in the art.